The present invention relates to a method for erasing data stored in a non-volatile semiconductor memory integrated circuit (IC), and more particularly to a method for erasing the data by irradiating an X-ray beam to memory cells.
Commonly used memory cells of a non-volatile memory IC which uses insulated gate transistors are ones in which data are stored by injecting charges from a semiconductor substrate into a trap of charge which is present in a single-layer or multi-layer gate insulation layer by tunneling effect and ones in which avalanche break-down is caused at a P-N junction (between a source or drain and a substrate) of a semiconductor substrate surface in a floating gate of a semiconductor layer, e.g. poly-silicon layer, burried in a gate insulation layer so that high energy electrons or holes generated by the avalanche break-down are injected into the floating gate to store data.
In those non-volatile memory IC's, especially in electrically writable (programable) read-only memories (called EPROM), it is a currently used method to irradiate an ultraviolet ray to the memory cells prior to writing new data in order to impart energy to electrons injected in the floating gates to discharge the electrons to the underlying semiconductor substrate through an electric barrier of a gate oxide layer in order to erase the data.
However, since the ultra-violet ray has to enter into the memory cells in order to erase the data by the ultra-violet ray, the IC chip of the semiconductor memory is packaged by a lower casing (or a support substrate) of ceramic material and an upper casing having a glass window. As a result, in spite of the reduction of manufacturing costs of the IC, the cost of the entire memory device is high because of the package material cost and the assembling cost. In addition, in the ultra-violet ray erasing method, if opaque particles deposit on the IC chip, especially on the memory cells, the data in those memory cells may not be erased completely. Furthermore, the packaging process is complex and many parts are included so that many of the manufacturing cycles take longer time than those for a conventional plastic mold IC.
The present invention is directed to a method for erasing data of the non-volantile memory IC which eliminates the difficulties encountered in the ultra-violet ray erasing method, and it is an object of the present invention to allow to package the non-volatile memory IC by plastic material and to market a low cost IC.
It is another object of the present invention to simplify the packaging process of the non-volatile memory IC and to allow to market the non-volatile memory IC which requires shorter manufacturing time.
It is another object of the present invention to provide a method for uniformly erasing data in all of the memory cells irrespective of presence or absence of programmed data.
It is a still further object of the present invention to provide a method for completely erasing data irrespective of fine particles which may deposit or the surfaces of the memory cells.